2 batteries in parallel

[airdvr1227]

Is it OK to just hook 2 batteries together? I leave my rig at a wet slip that is an hour away with no available electric and want to really beef up the available battery power for the automatic bilge.

I know about switches and isolators. Can 1 alternator charge 2 batteries without damging the alternator or the batteries?

 

[Mordekai]

Re: 2 batteries in parallel

You'd have to see what is the charging amp of your alternator most outboard aren't as high as inboard or i/o engines.

My 3.0L mercruiser charges both of my battery pretty well when used with the bluesea ACR since the alternator puts out 60 charging amp.

 

[Silvertip]

Re: 2 batteries in parallel

Two batteries can be used in parallel without using an isolator, ACR, combiner or dual battery switch IF (note the IF) the boat is used frequently. If allowed to sit for long periods of time the weaker battery in the pair will draw down the stronger battery until the two are equal. Even two, identical, brand new batteries will not have identical characteristics although they are close. The alternator doesn't care how many batteries are in parallel because it sees them as one giant 12 volt battery. The only issue is the charging output of the alternator and whether or not you have a trolling motor or any other very high current draw items on the boat. If you do, the available charging current may not be able to meet the demand. Without high current draw items there will be no issue.

 

[airdvr1227]

Re: 2 batteries in parallel

Thanks all for the quick replies. I'm definitely having charging issues and I suspect my alternator. I'm going off of my in-dash voltmeter so I bought an external meter to double check but as I was running the other evening, the gauge was running below 12 volts and as I adjusted trim the engine would stumble. I shut off everything I could at that point but there wasn't enough left in the batteries and she stumbled all the way back to the dock.

I figured that I was running on battery only but when I got back to the dock I shut her off and she started right back up so that was a bit confusing to me.

 

[Silvertip]

Re: 2 batteries in parallel

You definitely have a charging (or battery) issue. When you hit the trim you caused the voltage to drop to a point where the ignition system couldn't function. Before replacing an expensive alternator, make sure all of the battery cables are clean and tight on both ends. make sure the battery is capable of actually "holding" a charge. A bad battery simply won't accept a charge. Charge it with a portable charger and then have it load tested at any auto store. Take the alternator with you. Also be aware that alternators can be rebuilt by any auto/electric shop (starter alternator rebuilder).

 

[airdvr1227]

Re: 2 batteries in parallel

Along the same lines let's assume I need an alternator. What is it that makes an alternator a 'marine' alternator? I've owned alot of vehicles in my time and I've never seen sparks coming from any alternator.

 

[NYBo]

Re: 2 batteries in parallel

Along the same lines let's assume I need an alternator. What is it that makes an alternator a 'marine' alternator? I've owned alot of vehicles in my time and I've never seen sparks coming from any alternator.

Whether you've ever seen them or not, an alternator does produce sparks. A marine alternator is shielded to prevent fumes from reaching inside the case. Take a look at the back of this one, contrasted with the large openings in a comparable auto alternator:

 

[spikeitaudi]

Re: 2 batteries in parallel

Great Thread here. Learning alot. Thanks for asking airdvr.

 

[Silvertip]

Re: 2 batteries in parallel

There are slip rings inside an alternator that function in much the same way brushes do in a starter or generator. Since the armature is spinning, how do you think current flows from that spinning armature to the field which is stationary? Yup -- through slip rings and brushes. Starters make sparks, generators make sparks, and alternators make sparks.

 

[DianneB]

Re: 2 batteries in parallel

Sounds like you have the problem in hand (alternator) but I was going to comment on the two batteries topic.

I have two batteries in my 24 foot cruiser (a house battery and an engine battery) with an ACR between them and I have a bilge pump wired to each battery for redundancy. My boat is old, seeps water, and is 2 hours away in a slip so I want to make sure she stays dry. I use the BOTH position on the battery switch for starting and she starts more easily than on one battery alone.

 

[airdvr1227]

Re: 2 batteries in parallel

This is getting interesting. To double check the gauge on my dash I used a voltmeter at the battery. According to the meter at idle the alternator is putting out about 13.5 volts which should be normal right? WHen I start adding electical load the in-dash voltmeter drops to below 12 and is telling me things are discharging, yet nothing changes at the battery...still around 13.5v.

I'm wondering about the actual charge on the batteries and whether that is affecting the in dash gauge v. the meter.

 

[EricJRW]

Re: 2 batteries in parallel

Have you looked into a solar panel for keeping the batteries topped up?

It's getting pretty common. Harbor Freight has decent prices, but I also heard eBay is a good choice. Apparently there is a Japanese company that ships cars with solar chargers on the roof. When they get to the dealer they are removed and end up on eBay.

PS. Just came across this thread, it's worth a read... Panels may not be as good of an idea as I thought...

http://forums.iboats.com/showthread.php?t=421982

 

[dbkerley]

Re: 2 batteries in parallel

You guys are really hitting the target here. 2 batteries with a good isolation switch so you can choose to run one the other or both and also have the ability to disable it down to nothing but the automatic bilges just in case you get a faulty circuit while you are away.

One other point, is a standby fuse block. I have one epoxied to the underside of my dash. So, if I lose power through one of my breakers or I have to quickly add or restore an emergency circuit I can just pull the connector and temporarily attach it to the unused fuse block.

Nothing works better than redundancy on the water.

 

[Silvertip]

Re: 2 batteries in parallel

Marine alternators are "ignition protected" meaning they are sealed so the sparkes generated by them do not ignite gas fumes in the bilge of the engine bay. Yes - starters and alternators do spark internally at the brushes. If there is any opening to the atmosphere there is a risk of explosion.

 

[airdvr1227]

Re: 2 batteries in parallel

Ok...here's the deal now. I had both batteires checked at autozone and both are good. Re-did all of the battery connections. Hooked both up in parrallel. I have 13.8v at idle and 14.5v at 1500 rpm. My in-dash voltmeter isn't agreeing. I might have a problem with my running lights. WHen I turn them on is when the voltmeter in the dash drops way down, but I also noted that all of the other gauges change off that switch. Just slightly but they do change. SHort in my running lights wiring somewhere?

 

[DianneB]

Re: 2 batteries in parallel

It could be a heavy load in your nav light (which I assume are fused) but more likely to be a problem (poor connection) in the power feed or ground between your battery and the console.

 

[stylesabu]

Re: 2 batteries in parallel

your running lights may be a problem, but there not on all the time.when you are running every component should have same voltage. using you handheld volt meter start checking voltages.i would start at control box. if you have 13.5 volts at battery you should have that at the radio, all the switches,the volt meter the tach, everything.

 

[Silvertip]

Re: 2 batteries in parallel

What may be happening is that the +12 volt feed from the battery to the fuse panel is sized too small. Therefore when the engine is running, the batteries show they are being charged when voltage is checked "at the battery". However, as loads are applied, such as the docking lights or other high current draw items, that extra power is drawn all the way from the battery and the dash voltmeter is measuring voltage way downstream. Wire that is too small for that long a run causes voltage drop which is what you are experiencing. Your batteries and alternator are fine. The feed to the fuse panel is not.

 

[Eric 3.7LX Alpha 1]

Re: 2 batteries in parallel

What may be happening is that the +12 volt feed from the battery to the fuse panel is sized too small. Therefore when the engine is running, the batteries show they are being charged when voltage is checked "at the battery". However, as loads are applied, such as the docking lights or other high current draw items, that extra power is drawn all the way from the battery and the dash voltmeter is measuring voltage way downstream. Wire that is too small for that long a run causes voltage drop which is what you are experiencing. Your batteries and alternator are fine. The feed to the fuse panel is not.

Ditto on this hypothesis.

I read that the alternator output wire is best run to the dash instead of the batteries, because it is there where the accessories are drawn from. With a 3 wire alternator the sense wire is from the dash area which will tell the regulator to adjust output based on accessory draw. The alternator output wire is run to a terminal block at the dash, and wire to battery is now just to charge the battery instead of feeding the accessories. The battery actually work better with a smaller gauge wire to them to allow a slower charge.
This would require an adequate sized wire to be added from the alt. to the dash which would feed the accessories directly off the terminal block.

This is not how my boat is wired but have read it is better than the standard way of alt to the starter, to the battery to the console(cockpit).

Here is the article on it, it focuses on autos but the charging system is similar:
http://madelectrical.com/electricaltech/onewire-threewire.shtml
Any opinions??

 

[bruceb58]

Re: 2 batteries in parallel

This would require an adequate sized wire to be added from the alt. to the dash which would feed the accessories directly off the terminal block.

Would also require an equally large sized wire to be run from the dash back to the battery. I don't see the point in it. If you run a sufficiently large wire from the battery to the dash, You should be getting negligible voltage drop already. You would also have to run this wire through a breaker. What happens if the breaker blows? If the breaker blows, you just wiped out your alternator becuase it now has no load. That is unless this is an additional wire.